Hydraulic control system

ABSTRACT

A hydraulic control system including a two stage valve system which is suitable to be used as one of a series of similar valve systems is described. The second or main stage valve is a spool valve which includes an open center passageway, that is, one which, at the center or neutral position of the spool, is open and which is connected as a link in a hydraulic path between a pump and a reservoir. The second stage valve also includes flow control passageways which make the rate of flow of fluid to the load proportional to the rate of flow of fluid through the first stage. The first stage valve may be a spool type valve or a poppet type valve.

United States Patent [111 3,742,980 B ers Jr. Jul 3, 1973 HYDRAULIC CONTROL SYSTEM Primary Examiner Henry T. Klinksiek [75] lnventor: James Otto Byers, Jr., Manchester, Amman Mlller Attorney-Louis Etlmger [73] Assignee: Sanders Associates, Inc., Nashua,

[57] ABSTRACT [22] Filed, Apn 3 1972 A hydraulic control system including a two stage valve system which is suitable to be used as one of a series of PP No: 240,628 similar valve systems is described. The second or main stage valve is a spool valve which includes an open cen- 52 us. Cl. 137/625.62, l37/625.69 terpassagewayflhatistohe whichiathe cememrhew 51 Int. Cl. F16k 11/00 POsition of the SPOOL is P and which is 58 Field of Search 137/625.62, 625.6, as a link in a hydraulic Path between 8 P 137/6256), 62564 62568, 59614 and a reservoir. The second stage valve also includes flow control passageways which make the rate of flow [56] References Cited of fluid to the load proportional to the rate of flow of fluid through the first stage. The first stage valve may UNITED STATES PATENTS be a spool type valve or a poppet type valve. 3,339,584 9/1967 Long l37/625.69

9 Claims, 6 Drawing Figures INPUT SIGNAL jPAIENIEII 3W5 3.742.980

ET 1 U? 3 Pp INPUT INPUT INPUT SIGNAL I SIGNAL 2 SIGNAL 3 O O PILOT PILOT PILOT STAGE I STAGE 2 STAGE 3 MAIN MAIN MAIN VALVE] VALVE 2 N 43 VALVE 3 25 2s 27 x 2e 2] L. I I J L. J l l- 22 22 LOAD I LOAD 2 H48 LOAD 3 PP pp 3] I 23 32 33 PRESSURE PRESSURE BOOSTER REDUCER P? 21 P A 21 FIG 2 FIG 5 HYDRAULIC CONTROL SYSTEM FIELD OF THE INVENTION BACKGROUND OF THE INVENTION Many itemsof mobile equipment, such as earth moving machinery and fork lift trucks and the like, have numerous components, each of which must be individually controllable. Such control is frequently obtained by one or more hydraulic actuators for each component together with a separate valve for each actuator. The controls of each valve are preferably arranged near to each other so as to be readily manipulated by the operator of the equipment. Typically, some or all of the valves are arranged side by side in a so-called stack. In such an arrangement, each valve is manually operated spool type valve with the usual lands and ports to constitute a four way valve and in addition includes lands and ports arranged to constitute a fluid passageway which is substantially open when the valve spool is in its neutral position with the load ports closed but which is variably occluded as the spool is displaced in either direction from its neutral position. The passageways of all of the valves are connected in series between a pump and a reservoir. These valves are often referred to as open center valves and, in the neutral position, fluid flows freely through them and the pump is substantially unloaded. An additional passageway or chamber, sometimes referred to as the high pressure carry over, is connected to the high pressure side of the pump and, through individual check valves, to the pressure ports of the four way sections of each valve.

If all of the valves are initially in their neutral positions and the operator desires to actuate one of the devices, he displaces the appropriate spool from the neutral position. This first connects the actuator from the pressure and tank ports of the valve but, in general, fluid flow will not start immediately because the pressure in the high pressure carry over chamber is quite low. Reverse flow from the load is prevented by the aforementioned check valve. As the operator displaces the spool further, the aforementioned series passageway will be partially closed, thereby allowing the pump to build up pressure in the high pressure carry over chamber. When this pressure has risen sufficiently, fluid will flow to the selected actuator. Further motion will increase the flow rate. Should the operator require flow to two or more devices simultaneously, the second spool is shifted until the pressure and tank ports are connected to that actuator. If the second valves load is less than the first, then flow will start at once when the port opens, and will increase as this spool is displaced further. If the load on the second spool is greater than on the first, it will be necessary to restrict the flow from the pump to the tank further with the second spool, while decreasing the opening to the load port on the first spool in order to build up the pressure in the high pressure carry over chamber sufficiently to overcome the load on the second spool. After flow starts to the second spool, the rate may be increased by shifting the spool further. By this means, the total pump flow can be divided to-as many actuators as the operator desires. The flow not required returns to tank after passing through the passageways in each of the valves. The only limitation on the number of valves in use or the metering of flow to any actuator is the skill of the operator.

Although valves as briefly described above are widely used, they are subject to certain limitations. For example, the manual operation of the valve imposes certain limitations on the locations of the controls. It would be desirable in some situations to replace such manual valves with electric remote control valves, and although such valves are known, hitherto they have been quite expensive. As another example, it has been found that in many cases only about 10 or 20 percent of the total range of movement of the control is effective to vary the rate of flow of fluid to the load. This is because it is often necessary to move the controller through half of its range before the pressure is built up sufficiently to allow any fluid at all to flow to the load. Then the flow may be controlled from a minimum on up throughout the next 10 20 per cent of movement but beyond this point little or no increase of fluid flow is obtained. There is plenty of room for improvement in this respect.

It is a general object of the present invention to provide an improved hydraulic control system.

Another object is to provide a remotely controlled valve system suitable for use as one unit in a multiple unit installation.

Another object is to provide a remote control valve system having improved control of the flow of fluid to the load.

SUMMARY OF THE INVENTION Briefly stated, a valve system incorporating the present invention comprises a two stage arrangement in which the main valve includes an open center passageway and the usual four way land and port arrangement modified to incorporate flow control passageways through which fluid flow from the first stage passes to the load and, in so doing, positions the main spool to make the total flow to the load substantially proportional to the input signal to the first stage. This input signal is an electric signal to a torque motor which controls the first stage valve. By this arrangement, very close control is achieved since a change in the load being controlled and/or a change in the requirements of other valves merely causes an automatic repositioning of the spool to compensate for such changes.

DESCRIPTION OF PREFERRED EMBODIMENT For a'clearer understanding of the invention, reference may be made to the following detailed description and the accompanying drawing in which:

FIG. 1 is a schematic block diagram of a hydraulic control system incorporating the present invention;

FIG. 2 is a fragmentary schematic diagram showing a preferred arrangement for maintaining the relative pressures of the fluid supplies to the pilot and main stages;

FIG. 3 is a fragmentary schematic diagram showing another arrangement for maintaining the relative pressures of the fluid supplies to the pilot and main stages;

FIG. 4 is a schematic diagram of one of the units shown in FIG. 1 including the details of the main valve;

FIG. 5 is a schematic diagram of a preferred form for the first, or pilot, stage; and

FIG. 6 is a schematic diagram of another form of a first, or pilot, stage which may be used.

Referring first to FIG. 1, there are shown three valve systems incorporating the present invention. Each system comprises a load to be actuated, a main valve, and a pilot stage. All of the main valves are connected to be supplied with fluid under pressure from a single pump 21, which may conveniently be a positive displacement pump, which draws fluid from a reservoir or tank 22 and delivers it through a conduit 23 to a junction 24. Each of the main valves in an open center spool and cylinder valve and each includes a passageway therethrough, which, when the spool is in its neutral position, is substantially unobstructed but which is variable occluded as the spool is moved in either direction from its neutral position. The junction 24 is connected by means of a conduit 25 to the inlet of this passageway in the first valve. The outlet of this passageway is connected by means of a conduit 26 to the inlet of the corresponding passageway in the second valve and the outlet thereof in turn is connected through a conduit 27 to a similar inlet of the third valve the outlet of which is connected through a conduit 28 to the tank 22. The junction 24 is also connected to a High Pressure Carry Over (HPCO) chamber or passageway 29 which in turn is connected to the usual fluid pressure input ports of all of the main valves. When all of the valves are in their neutral positions, the pressure input ports are closed but, since fluid flows practically unimpeded through the open center passageways, the pressure in chamber 29 is quite low.

Each of the valve systems includes a first, or pilot stage which is supplied through a conduit 31 with fluid at a pilot pressure (PP) which must be a fixed amount higher than the pressure supplied to the main valves through the chamber 29. This pressure differential may be maintained in any of several ways, one simple way which is preferred at present is shown in FIG. 2 and comprises a pressure booster 32 connected between the junction 24 and the pilot pressure conduit 31. Alternatively, as shown in FIG. 3, the pilot pressure conduit 31 may be connected directly to the output of the pump 21 while the conduit 23 is broken into two sections 23' and 23" which are interconnected by a pressure reducer 33.

The operation will be described briefly using the second valve system for illustrative purposes. This system includes a first or pilot stage 36 which is connected to the pilot pressure conduit 31 over a branch conduit 37. The first stage also includes a force motor or a torque motor (not shown separately in FIG. 1) which receives an input signal over an electrical path 38. The first stage 36 also includes a valve which is controlled by the force motor in accordance with the input signal to generate a corresponding flow of fluid in one or the other of conduits 41 and 42, according to the sense of the input signal. As previously mentioned, the conduits 26 and 27 are connected to the open center passageway of the main valve 43. This valve receives its principal pressure supply from the chamber 29 through a conduit 44 and is connected to the tank by means of a conduit 45. Conduits 46 and 47 connect the main valve 43 to its load 48.

The fluid flow in one of the control conduits 41 or 42 displaces the spool of the main valve 43 which, as will be more fully explained, partially obstructs the open center passageway thereby increasing the pressure in the chamber 29. At the same time, the valve 43 interconnects conduits 44 and 45 with load conduits 46 and 47 in the proper sense and proper amount to control the load 48.

Referring now to FIG. 4, the valve 43 includes a valve body 51 formed to define a hollow cylinder 52 in which is a valve spool indicated generally by the reference character 53. The spool 53 is shorter than the cylinder 52 thereby leaving end spaces 54 and 55 at opposite ends of the cylinder. At the right end of the spool, a compression spring 56 cooperates with a pair of brackets 57 and 58 and an extension 59 of the spool to return the spool to the neutral position shown in FIG. 4 in the absence of an input signal. The valve body 51 is also formed to define two annular grooves or ports 61 and 62 near the center of the hollow cylinder, which ports are connected to the conduits 26 and 27 respectively. The spool 53 is formed to define two lands 63 and 64 cooperating with the grooves 61 and 62 and positioned so that, in the neutral position shown in the drawing, there is a substantially unimpeded passage from the conduit 26 through the port 61, the interior of the cylinder 52 and the port 62 to the conduit 27. It is apparent that when the spool 53 is displaced in either direction from its neutral position, this passageway will be partially occluded.

The valve body 51 is also formed to define two ports 65 and 66, on opposite sides of ports 61 and 62, respectively, which are connected to conduits 46 and 47 respectively. The spool 53 is formed to define lands 67 and 68 which cooperate with the ports 65 and 66 and, in the neutral position of the valve shown, completely occlude these ports. The spool 53 is also formed to define lands 71 and 72 respectively at opposite ends thereof which serve to separate the end spaces 54 and 55 from the remainder of the interior of the cylinder 52. A pair of conduits 45 (shown in FIG. 1 as a single conduit) connects the tank 22 with the interior of the cylinder 52 between the lands 67 and 71 and also between the lands 68 and 72. The conductor 44, from the high pressure carry over chamber 29, is connected through two resistors 73 and 74 to the conduits 41 and 42 respectively which, in turn, communicate with the end spaces 54 and 55 respectively. The conductor 44 is also connected toa check valve 75 which in turn is connected to conduits 76 and 77 which in turn are connected to the interior of the cylinder 52 in the region between the lands 63 and 67 and in the region between the lands 64 and68. The valve 75 is connected to pass fluid from the conduit 44 to the conduits 76 and 77 but to prevent passage of fluid in the opposite direction.

The spool 53 is also formed to define two longitudinal passageways 81 and 82 respectively communicating with the end spaces 54 and 55 respectively and extending approximately parallel to the axis of the spool and terminating in radial slots 83 and 84 respectively. These slots are formed adjacent to those faces of the lands 68 and 67 respectively which are exposed to the pressure from the conduits 77 and 76. More particularly, the radial slots 83 and 84 are substantially rectangular in shape. One wall of these slots is defined by the face of its associated land. The other three walls are within a boss fastened to the face of its associated land. The slots 83 and 84 are conveniently referred to as flow control slots and are similar to and for the same purpose as the slot 46 and chamber 49 of US. Pat. No. 3,561,488. In the patent, two diametrically opposed slots are used whereas in the present application only one slot, extending along a single radius, is sued. However, the mode of operation is identical in both cases. The longitudinal passageways 81 and 82 contain check valves 85 and 86 respectively. These check valves permit the flow of fluid from the end spaces towards the radial slots but prevent flow of fluid in the reverse direction. Each is shown schematically as a ball because a simple ball type check valve is entirely suitable.

Initially, in the absence of an input signal, the parts will be in the positions shown in FIG. 4. The pressure in the conduit 44, denoted P1, is transmitted through the restrictors 73 and 74 to the end spaces 54 and 55, respectively. This pressure is comparatively low because, as previously noted, there is a very low resistance path through all the valves from the pump to tank.

When it is desired to move the load 48, an input signal is applied over the conductor 38 to the pilot stage 36 which responds by initiating a flow of fluid in one of the conduits 41 or 42. For illustrative purposes it will be assumed that the flowstarts in conduit 42. Since the pressure PP is higher than that of P1, fluid will initially flow through the restrictor 74 and into the conduit 44. Such a flow of fluid causes a pressure drop to appear across the pilot stage 36 and another pressure drop to appear across the restrictor 74. As the flow of fluid increases, the pressure in the conduit 42 and the end space 55 will rise. When the pressure has risen to equal Pl plus enough more to overcome the preloading of the spring 56, the spool 53 will be displaced to the left. This will open the control slot 84 to the groove 65 and will also open the interior of the cylinder between the lands 63 and 67 to the groove 65. However, the pressures of both P1 and that in the end space 55 are comparatively low and may in fact be lower than the back pressure existing in the load 48. If so, no flow of fluid will occur immediately and reverse flow through the longitudinal passageway 82 will be prevented by the check valve 86 while reverse flow to the conduit 44 will be prevented by the check valve 75. Accordingly, the spool 53 will be moved still further to the left and the central passageway between the grooves 61 and 62 will be further occluded thereby raising the pressure in the high pressure carry over chamber 29 and correspondingly raising the pressure PP at the pilot stage 36. Eventually, the central passageway will be sufficiently occluded to raise the pressure P1 sufficiently to overcome the pressure in the load at which time fluid will start to flow from the control slot 84 to the load and also from the interior of the cylinder over the land 67 through conduit 46 to the load. As the flow through the control slot 84 increases, the pressure drop across the pilot stage will also increase thereby eventually lowering the pressure in the end space 55 to equal P1 plu's whatever additional pressure is necessary to overcome the preloading of the spring 56. At this point, the spool 53 will come to rest. The rate of flow of fluid through the slot 84 is almost exactly equal to the rate of flow of fluid from the pilot stage to the conduit 42. It is not quite the same because of the necessity of making the pressure in the end space 55 slightly larger than P1 in order to overcome the small preloading of the spring 56, and second, because there is a small flow of fluid through the restrictor 74. This flow remains constant'but in most cases is very small indeed compared to the other rates of flow in the system. At the same time, there will be a much larger flow of fluid to the load from the conduit 44 around the land 67 and this flow will be almost exactly proportional to the flow through the control slot 84.

Referring now to FIG. 5 there is shown a preferred form for the first or pilot stage 36. There is shown a torque motor 91 which is arranged to displace a member 92 to the left or right in accordance with the sense of an input signal. The first stage 36 also includes a valve including a valve body 93 formed to define a chamber 94 containing the member 92. The valve body 93 is also formed to define first and second generally cylindrical coaxial apertures communicating with the chamber 94 at opposite sides thereof and extending to the exterior of the valve body 93. generally cylindrical plugs 95 and 96 are threaded into these apertures. The plugs are identical and only one will be described in detail. The plug 95 is formed to define a central axis bore 97 extending from the end of the plug adjacent the chamber and along the axis for a portion of its length. The plug is also formed to define a transverse passageway 98 which intersects the bore 97. The plug 95 is also formed to define an annular groove 99 around the outside surface thereof which intersects the transverse passageway 98 at each end thereof. The valve body 93 is also formed to define a passageway 101 extending from the groove 99 to the exterior of the valve body where it is connected to the conduit 41. The conduit 37 carrying the pilot pressure source is connected to an internal passageway 102 formed in the valve body 93 and which extends into the chamber 94. The rim of the bore 97 at its intersection with the chamber 94 is formed to define or has impressed therein a valve seat 103 which cooperates with a poppet valve 104 positioned within the chamber 94 so as to be urged by the pressure of any fluid therein into engagement with the seat 193.

The plug 96 is substantially identical to the plug 95 and cooperates with a poppet valve similar to the valve 104. It is preferred that the mechanical connection be tween the torque motor and the poppet valve be such that displacement of the member 92 to the left moves only that valve associated with the plug 96 while movement of the member 92 to the right displaces only the poppet valve 94. This is conveniently accomplished by making the member 92 U shaped and providing apertures in the legs through which the stems of the poppet valves pass with clearance and by securing these stems on the inside of the U shaped bracket by means of a cross member 106 such as a cotter pin.

As previously mentioned, the plug 95 is threaded into the body 93 and it is initially adjusted to make the relative positions of the valve 104 and the seal 103 such that flow is just ready to start should the member 92 be displaced to the right to pull the valve 104 off its seat. The plug 96 is similarly adjusted.

When current is applied to the torque motor 91, the member 92 will gradually overcome the pressure P acting on the poppet valves. When this pressure force has been matched, flow will start to conduit 41 or 42 depending upon the direction of the current of the torque motor. Any further increase in current to the torque motor will cause one of the poppets to move off its seat and to allow fluid to flow at a rate depending upon the opening at the poppet and the pressure difference between that in the chamber 94 and that in the conduit 41 or 42. The torque motor 91 is designed so as to cause the member 92 to move in a direction determined by the sense of the input current and a distance exactly proportional to the magnitude of the input current.

The details of the pilot stage illustrated in FIG. are not part of the present invention. This stage is more fully described and claimed in the copending application of James Otto Byers, Jr. filed concurrently herewith and entitled Poppet Type Proportional Valve Pilot Stage. This valve stage has been found to be particularly suitable for use in connection with the system of the present invention.

Another suitable first or pilot stage is shown in FIG. 6. This stage will not be described in great detail since it consists of a force motor and a conventional three way spool valve. Such valves are well known and have been described in many places in the literature. One example is in the aforementioned US. Pat. No. 3,561,488 wherein FIG. 1 shows a valve very similar to that presently illustrated in FIG. 6 of the present application. Briefly stated, the pressure source from the conduit 37 is connected to a port 111 which, in the neutral position, is completely occluded by a land 112. Displacement of the valve spool including the land 112 in either direction by the force motor 113 opens a passageway from the port 111 to either the load conduit 41 or the load conduit 42.

From the foregoing it will be apparent that Applicant has provided a novel and useful control system. if the load on one of the valves should change, the: spool of the affected valve will automatically be adjusted to keep the flow rate from the control conduit 41 or 42 (whichever one is then active) into the load substantially constant without any attention on the part of an operator. It follows, of course, that the total flow is likewise held constant. If, while one valve is operating in a stable condition, another valve should be brought into operation in order to actuate a second load; the spool 52 will again automatically adjust itself to maintain the flow to the load constant. it is apparent that the valves will automatically adjust themselves to all possible pressure relationships, within the capabilities of the system, such that the flows to any load will not be appreciably affected by changes in pressure at other loads.

It is to be noted that, because of the above noted automatic operation, a very large range of input signals is effective to generate the desired rate of flow to the load. This is in sharp contrast to the prior valves wherein, as previously mentioned, adjustment is quite critical since only about l0 or percent of the range of movement of the controller is actually effective to vary the flow rate to the load.

It is also to be noted that since the travel of the spool 53 before flow starts to the load is not critical, the overlap of the lands 67 and 68 can be the same as on more conventional manual valves.

Although a preferred embodiment of the invention has been described in considerable detail for illustrative purposes, many modifications will occur to those skilled in the art. it is therefore desired that protection afforded by Letters Patent be limited only by the true scope of the appended claims.

What is claimed is:

l. A valve system having a spool movable within a cylinder and including first and second lands arranged to variable expose first and second load ports to fluid under pressure and to tank in a sense and in an amount determined by an input signal, and which includes third and fourth lands arranged to differentially occlude third and fourth internally connected ports respectively, said third and fourth ports being connected externally to form a part of a fluid passageway originating at a source of fluid under pressure and terminating in the tank, and in which said lands are positioned relative to each other and to said ports so that in a neutral position of said spool said first and second lands completely occlude said first and second ports while said third and fourth ports are substantially unobstructed by said third and fourth lands, and which includes a fluid passageway having a check valve therein which connects said source of fluid under pressure through said check valve to the interior of said cylinder adjacent to said first and second lands, characterized in that said source is connected through first and second restrictors to opposite end spaces in said cylinder and in which said spool is formed to define first and second longitudinal passageways each connected to one of said end spaces and to a radial passageway adjacent to that face of said first and second lands respectively, which is exposed to the fluid pressure within said cylinder, and which includes a pilot state responsive to said input signal for generating a corresponding rate of flow of fluid and conducting said flow to one or the other of said end spaces.

2. A valve system in accordance with claim 1 in which each of said longitudinal passageways includes a check valve arranged to allow passage of fluid from its associated end space to its associated radial passageway and to prevent the flow of fluid in the reverse direction.

3. A valve system in accordance with claim 1 in which each of said radial passageways is substantially rectangular in cross section and one wall of which is the face of its associated land.

4. A valve system in accordance with claim 1 including a spring for urging said spool toward said neutral position.

5. A valve system in accordance with claim 1 in which said pilot stage includes a first stage valve for controlling the flow of fluid from an additional source into first and second conduits in accordance with an input signal and in which said first and second conduits are connected to opposite end spaces of said cylinder.

6. A valve system in accordance with claim 5 including a force motor for controlling said first stage valve in accordance with said input signal.

7. A valve system in accordance with claim 6 in which said first stage valve is a three-way valve including a spool moveable within a hollow cylinder.

8. A valve system in accordance with claim 6 in which said first stage valve includes first and second poppet valves actuated by said force motor for controlling the flow of fluid to said first and second conduits.

9. A valve system comprising a source of fluid under pressure, a tank for receiving the fluid returned from the system, a valve housing formed to define a hollow cylinder, said cylinder being formed to define first and second ports connected to first and second load conduits respectively, a valve spool within said cylinder intermediate the end thereof so as to define first and second end spaces at opposite ends of said cylinder, said spool being moveable in each direction from a neutral position, said spool including first and second lands located to occlude said first and second ports when said spool is in its neutral position, said spool also including third and fourth lands located between said first and second lands and said first and second end spaces respectively, said housing being formed to include passageways for interconnecting said tank with the interior of said cylinder between said first and third lands and also between said second and fourth lands, said spool also including fifth and sixth lands both positioned intermediate said first and second lands, said housing being formed to define third and fourth ports cooperating with said fifth and sixth lands respectively, and positioned to be substantially open to the interior of said cylinder when said spool is in said neutral position and to be variably occluded by said fifth and sixth lands respectively as said spool is displaced in opposite directions from said neutral position, said spool and cylinder being formed so that said third and fourth ports are in communication with each other within said cylinder, said third and fourth ports being connected externally so as to form a portion of a passageway originating at said source and terminating at said tank, said housing being formed to define a chamber, a first fluid connection from said source to said chamber, a second fluid connection from the interior of said cylinder between said first and fifth lands to the interior of said cylinder between said second and sixth lands, a check valve hydraulically connected to allow flow of fluid from said chamber to said second fluid connection, and to prevent flow in the opposite direction, characterized in that said system includes a pilot stage having a torque motor and a valve responsive to the sense and magnitude of an input signal for selecting either a first or a second conduit and generating a variable fluid flow therein, said first and second conduits being connected to said first and second end spaces respectively, first and second restrictors connected between said chamber and said first and second end spaces respectively, said spool being formed to define first and second longitudinal passageways in communication with said first and second end spaces respectively, and also in communication with first and second radial passageways, said radial passageways being contiguous to those faces of said first and second of lands which are nearest to said fifth and sixth lands, and two additional check valves, one interposed in each of said longitudinal passageways so as to prevent the flow of fluid from said radial passageways to said end spaces but to permit a flow of fluid in the opposite direction. 

1. A valve system having a spool movable within a cylinder and including first and second lands arranged to variable expose first and second load ports to fluid under pressure and to tank in a sense and in an amount determined by an input signal, and which includes third and fourth lands arranged to differentially occlude third and fourth internally connected ports respectively, said third and fourth ports being connected externally to form a part of a fluid passageway originating at a source of fluid under pressure and terminating in the tank, and in which said lands are positioned relative to each other and to said ports so that in a neutral position of said spool said first and second lands completely occlude said first and second ports while said third and fourth ports are substantially unobstructed by said third and fourth lands, and which includes a fluid passageway having a check valve therein which connects said source of fluid under pressure through said check valve to the interior of said cylinder adjacent to said first and second lands, characterized in that said source is connected through first and second restrictors to opposite end spaces in said cylinder and in which said spool is formed to define first and second longitudinal passageways each connected to one of said end spaces and to a radial passageway adjacent to that face of said first and second lands respectively, which is exposed to the fluid pressure within said cylinder, and which includes a pilot state responsive to said input signal for generating a corresponding rate of flow of fluid and conducting said flow to one or the other of said end spaces.
 2. A valve system in accordance with claim 1 in which each of said longitudinal passageways includes a check valve arranged to allow passage of fluid from its associated end space to its associated radial passageway and to prevent the flow of fluid in the reverse direction.
 3. A valve system in accordance with claim 1 in which each of said radial passageways is substantially rectangular in cross section and one wall of which is the face of its associated land.
 4. A valve system in accordance with claim 1 including a spring for urging said spool toward said neutral position.
 5. A valve system in accordance with claim 1 in which said pilot stage includes a first stage valve for controlling the flow of fluid from an additional source into first and second conduits in accordance with an input signal and in which said first and second conduits are connected to opposite end spaces of said cylinder.
 6. A valve system in accordance with claim 5 including a force motor for controlling said first stage valve in accordance with said input signal.
 7. A valve system in accordance with claim 6 in which said first stage valve is a three-way valve including a spool moveable within a hollow cylinder.
 8. A valve system in accordance with claim 6 in which said first stage valve includes first and second poppet valves actuated by said force motor for controlling the flow of fluid to said first and second conduits.
 9. A valve system comprising a source of fluid under pressure, a tank for receiving the fluid returned from the system, a valve housing formed to define a hollow cylinder, said cylinder being formed to define first and second ports connected to first and second load conduits respectively, a valve spool within said cylinder intermediate the end thereof so as to define first and second end spaces at opposite ends of said cylinder, said spool being moveable in each direction from a neutral position, said spool including first and second lands located to occlude said first and second ports when said spool is in its neutral position, said spool also including third and fourth lands located between said first and second lands and said first and second end spaces respectively, said housing being formed to include passageways for interconnecting said tank with the interior of said cylinder between said first and third lands and also between said second and fourth lands, said spool also including fifth and sixth lands both positioned intermediate said first and second lands, said housing being formed to define third and fourth ports cooperating with said fifth and sixth lands respectively, and positioned to be substantially open to the interior of said cylinder when said spool is in said neutral position and to be variably occluded by said fifth and sixth lands respectively as said spool is displaced in opposite directions from said neutral position, said spool and cylinder being formed so that said third and fourth ports are in communication with each other within said cylinder, said third and fourth ports being connected externally so as to form a portion of a passageway originating at said source and terminating at said tank, said housing being formed to define a chamber, a first fluid connection from said source to said chamber, a second fluid connection from the interior of said cylinder between said first and fifth lands to the interior of said cylinder between said second and sixth lands, a check valve hydraulically connected to allow flow of fluid from said chamber to said second fluid connection, and to prevent flow in the opposite direction, characterized in that said system includes a pilot stage having a torque motor and a valve responsive to the sense and magnitude of an input signal for selecting either a first or a second conduit and generating a variable fluid flow therein, said first and second conduits being connected to said first and second end spaces respectively, first and second restrictors connected between said chamber and said first and second end spaces respectively, said spool being formed to define first and second longitudinal passageways in communication with said first and second end spaces respectively, and also in communication with first and second radial passageways, said radial passageways being contiguous to those faces of said first and second of lands which are nearest to said fifth and sixth lands, and two additional check valves, one interposed in each of said longitudinal passageways so as to prevent the flow of fluid from said radial passageways to said end spaces but to permit a flow of fluid in the opposite direction. 